1. Field of the Invention
The present invention relates to a polishing machine for polishing the surface of substrates such as wafers with high precision and a method thereof.
2. Description of the Related Art
As a result of advances in super-fine processing and the profileration of semiconductor devices in recent years, chemical and mechanical polishing (CMP) machines as processing means for polishing SOI substrates, semiconductor wafers comprising Si, GeAs or InP, wafers having an insulation film or a metal film on the surface in the production process of integrated semiconductor circuits and substrates for use in displays have been widely used.
Conventional CMP machines are described hereinafter referring to FIG. 8 and FIG. 9. FIG. 8 shows an example for polishing a wafer 1 using a polishing pad 2 comprising, for example, polyurethane having a larger diameter than the diameter of the wafer 1, wherein the wafer 1 as a polishing object is held with a wafer holder 3 by holding its polishing object face downward. This polishing pad 2 has an uneven or a porous surface. As shown in FIG. 9, the wafer 1 is rotated with a driving means (not shown in the drawing) along the direction indicated by an arrow. The polishing pad 2 is also rotated with a driving means (not shown in the drawing) along the direction indicated by an arrow. The polishing object face of the wafer 1 making contact with the polishing pad 2 is polished by a relative rotation between the wafer 1 and polishing pad 2 or by rotating either one of them. A polishing agent (slurry) is fed from a slurry feed means 5 for the purpose of improving the degree of polishing. The slurry is composed of, for example, an aqueous alkaline solution in which fine particles of SiO.sub.2, having a particle size on the order of a micron or sub-microns, are stably dispersed. The slurry is fed between the wafer 1 and polishing pad 2 from outside.
FIG. 9 is an example where a polishing pad 2 having a smaller diameter than the diameter of the wafer 1 is held with a polishing pad holder 6 to polish the wafer 1 fixed by holding its polishing face upward.
The slurry is fed from a slurry feed means (not shown in the drawing) connected to a small hole 7 provided at the polishing pad to the gap between the wafer 1 and polishing pad 2 through a small hole 7.
However, there are problems in the conventional type CMP machines described above that a sufficient amount of slurry is not retained between the wafer 1 and polishing pad 2, because a centrifugal force is generated when the wafer 1 or the polishing pad 2, or both of them, is rotated, thereby pushing the slurry fed between the wafer 1 and polishing pad 2 outward.
The foregoing discussion will be described in more detail. In the conventional type CMP machines shown in FIG. 8, it is difficult for the slurry to penetrate into the gap between the rotating wafer 1 and polishing pad 2 since the slurry is fed between the wafer 1 and polishing pad 2 from outside. Although the slurry is fed through the small hole 7 to feed it between the wafer 1 and polishing pad 2 at the initial stage in the conventional type CMP machines shown in FIG. 9, the slurry is thrown out of the gap between the wafer 1 and polishing pad 2 by centrifugal force.
Consequently, polishing is carried out while insufficient amount of the slurry is not retained between the wafer 1 and polishing pad 2 in the conventional type CMP machines shown in FIG. 8 and FIG. 9. This results in a decrease of the degree of polishing. Accordingly, even when a fresh slurry is fed in order to maintain a high degree of polishing, the amount of the slurry retained between the wafer 1 and polishing pad 2 remains decreased, thereby hindering the degree of polishing. The remaining slurry tends to be localized between the wafer 1 and polishing pad 2, thereby resulting in an uneven polishing when polishing is continued under this condition.
While the polishing object face of the wafer 1 is kept wet by retaining a sufficient amount of the slurry on the surface of the wafer 1, the polishing object face of the wafer 1 is liable to be dry, on the contrary, when a sufficient amount of the slurry is not retained on the polishing object face of the wafer 1.
Consequently, the polishing debris created during polishing is unexpectedly absorbed on the polishing object face of the wafer 1. For example, the fine particulate components of the slurry, especially the fine particles comprising SiO.sub.2 or Ce, are extremely liable to be absorbed on the wafer 1, and the fine particles once absorbed as described above are difficult to remove from the wafer 1.
The foregoing fine particles are coagulated by themselves or with the fine particles that are components of the slurry in a dry condition, forming large coagulation masses. The coagulation mass unexpectedly injures the wafer surface when polishing proceeds without removing the coagulation mass from the surface of the wafer 1.
A frictional heat would accompany polishing when a sufficient amount of the slurry is not retained between the wafer 1 and the pad 2. When the polishing object face of the wafer 1 involves semiconductor elements, the surface of the semiconductor elements experience a heat modification, causing deterioration of electric characteristics of the semiconductor device.
When the rotation speed of the wafer 1 or polishing pad 2 is increased in order to increase the degree of polishing or to improve productivity, larger centrifugal force is applied, consequently further reducing the amount of slurry between the wafer 1 and the polishing pad 2.
The unexpected frictional heat as hitherto described tends to also increase.
As hitherto described, a variety of unexpected phenomena are caused when a sufficient amount of the slurry is not retained between the wafer 1 and the polishing pad 2, thereby leading to a poor wafer quality.
When the polishing object face is a substrate for use in displays composed of a substrate for use in expensive highly integrated circuits such as a microprocessor or a thin film semiconductor, it is crucial to reduce the production cost to improve the yield of the substrate.
Much more slurry than necessary has been continuously fed during the polishing process in the conventional art for the purpose of solving the foregoing problems. However, this method imposes a large burden on the production cost.
Although conventional wafers have a diameter of 6 inches, the diameter of the wafer will be largely increased to 12 inches or more in the future. Consumption of the slurry increases with the enlargement of the wafer diameter, requiring reconsideration of new measures and methods for efficiently feeding the slurry.
A dust generated in the polishing process adheres again on the wafer to cause functional deterioration of the wafer. The dust scattered in the environment may also cause spreading of contamination all over the polishing machine or around the polishing machine, thereby requiring frequent a short term maintenance of the polishing machine or installation of the polishing machine in a clean environment. Therefore, efficient recovery of the generated dust is essential.
The object of the present invention is, based on the problems of the conventional art, to provide a measure or a method for retaining a sufficient amount of slurry between the wafer and polishing pad.